fix: Add validation and error handling to ReplaceFileContents task

[gidadomukhtar]

fix: Add validation and error handling to ReplaceFileContents task

• Validate source file exists before processing
• Validate replacement file exists if specified
• Add comprehensive error logging for file I/O failures
• Handle edge cases in replacement key parsing
• Detect and warn on duplicate replacement keys
• Wrap file operations in try-catch to prevent silent failures

This improves robustness by ensuring the task fails gracefully with clear error messages instead of leaving corrupted output files.

Fixes task failures when source or replacement files are missing.

[Copilot]

Pull request overview

This PR hardens the ReplaceFileContents MSBuild task by adding input validation and more explicit error handling/logging so missing files and I/O failures fail the build cleanly rather than producing corrupted/partial outputs.

Changes:

• Added source/destination/replacement-file validation and wrapped file operations in try/catch with explicit error logging.
• Improved replacement parsing to handle empty inputs and malformed lines more defensively.
• Added duplicate replacement-key detection with warnings (last value wins).

Suppressed comments (1)

build-tools/prep/ReplaceFileContents.cs:6

• using System.Text; appears unused in this file (no Encoding, StringBuilder, etc.). Since this PR already touches the using block, consider removing it to avoid unnecessary usings/warnings.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

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